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Lecture 8: Spiral Structure Spiral Structure from Stars and Gas Lecture 8: Spiral Structure Spiral structure from stars and gas • 1949 spiral structure first traced in Andromeda galaxy, M31 using OB stars and HII nebulae • 1951 spiral structure first demonstarted in the Galaxy by Morgan, Osterbrock and Sharpless (Yerkes Observ.) using OB stars and young associations, and showing parts of three spiral arms • The arms are: (a) the Perseus arm (~2 kpc out from Sun) (b) the Local or Orion arm (passing near Sun) (c) the Sagittarius arm (~2 kpc towards centre) Young Population I objects define the location of the galactic spiral arms The pitch angle is about 25º (angle between arm and a circle through the arm, centred on the galactic centre, G.C.) • Our Galactic disk does not appear solid. • It has spiral arms, much like we see in other galaxies like M51 • These arms are not fixed strings of stars which revolve like the fins of a fan. • They are caused by compression waves which propagate around the disk. • such waves increase the density of matter at their crests • we call them density waves • they revolve at a different speed than individual star orbit the Galactic center • Note how the spiral arms appear bluer M 51 compared to the bulge or the gaps between the arms. Spiral Galaxy M83 observed in both visible light and radio wavelengths. Spiral Arms • The compression caused by density waves triggers star formation. • molecular clouds are concentrated in arms…plenty of source matter for stars • short-lived O & B stars delineate the arms and make them blue & bright • long-lived low-mass stars pass through several spiral arms in their orbits around the disk Spiral structure 1- Spiral tracers • Spiral tracers are objects that are commonly found in spiral arms and so are used to trace spiral structure. • Photos in blue taken of nearby spiral galaxies such as M31 (Andromeda galaxy) show the bluish spiral arms distinctly. This appearance results from bluish supergiants, H II regions, and population I cepheids that lie in the spiral arms. • How to apply these tracers to our Galaxy to determine the layout of its spiral arms near the sun? It’s not a simple operation! • First, it requires an accurate and reliable technique for measuring the distance to each of the tracers. • Second, the dust obscuration restricts optical observation of starlight. • Third, the sun’s location in the plane gives us a poor vantage point for seeing the Galaxy’s spiral structure because we are forced to observe it edge-on rather than face-on. • How cepheid variables have been used to delineate spiral features? The cepheids have the advantage that their distances are easy to determine by the period-luminosity relationship. This technique is so important that you should know how it works. • A variable star is one whose luminosity changes with time. A light curve is a graph of the change in a star’s flux with time. A star whose light varies in a regular fashion is known as a periodic variable. • In 1912 Henrietta S. Leavitt noted a number of cepheid variable stars in the Small Magellanic Cloud. At the time, the distance to this galaxy was not known, so Leavitt could not find out the luminosities of these stars. She did recognize that their periods were related to their average fluxes. Because these stars all lie at the same distance from the earth, their luminosities must be related to their periods. For the cepheid variable, Leavitt found that a special relationship, the period-luminosity relationship, connects the luminosity of a cepheid variable to its period. Basically, it states that the longer the period of light variation of a cepheid, the more luminous it is. How to use the relationship to find distances to cepheids? 1- Find a cepheid (identifying it by its light curve) 2- Measure its period of light variation from peak to peak. 3- Find the star’s average luminosity from the period-luminosity relationship 4- Measure the star’s flux by telescopic observations. 5- Calculate its distance from the inverse-square law for light. You have the flux and luminosity, so you can find its distance. اﻟﺴﻄﻮع (Luminosity) • ﻳﺴﻤﻲ ﻋﻠﻤﺎء اﻟﻔﻠﻚ ﻣﻘﺪار اﻟﻄﺎﻗﺔ اﻟﺘﻲ ﻳﻄﻠﻘﻬﺎ ﺟﺮم ﻣﺎ ﻓﻲ اﻟﺜﺎﻧﻴﺔ ﺑﺎﻟﺴ ﻄﻮع وﻳﺮﻣﺰ ﻟﻪ ﺑﺎﻟﺮﻣﺰ L. ﻟﻜﻲ ﻧﺪرك آﻴﻒ ﻳﺴﺘﻄﻴﻊ اﻟﻔﻠﻜﻴﻴﻦ إﺟﺮاء ﺗﻠﻚ اﻟﻘﻴﺎﺳﺎت ﻳﺘﻌﻴﻦ أن ﻧﺪرس أ و ﻻً اﻟﻌﻼﻗﺔ ﺑﻴﻦ اﻟﺴﻄﻮع اﻟﻈﺎه ﺮي “ (brightness)وﻧﺮﻣﺰ ﻟﻪ ﺑﺎﻟﺮﻣﺰ B“ ﻟﻠﺠﺮم وﺑﻌﺪﻩ واﻟﺘﻲ ﺗﻌﺮف ﺑﻘﺎﻧﻮن اﻟﺘﺮﺑﻴﻊ اﻟﻌﻜﺴﻲ . ﻗﺎﻧﻮن اﻟﺘﺮﺑﻴﻊ اﻟﻌﻜﺴﻲ وﻗﻴﺎس ﺳﻄﻮع ﻧﺠﻢ • ﻳﻨﺺ ﻗﺎﻧﻮن اﻟﺘﺮﺑﻴﻊ اﻟﻌﻜﺴﻲ ﻋﻠﻰ أن اﻟﺴﻄﻮﻋﻴﺔ اﻟﻈﺎهﺮﻳﺔ ﻟﺠﺮم ﺗﺘﻐﻴﺮ ﻣﺘﻨﺎﺳﺒﺔ ﻋ ﻜ ﺴ ﻴ ﺎً ﻣﻊ ﻣﺮﺑﻊ ﺑﻌﺪﻩ . اﻟﻔﻮﺗﻮﻧﺎت اﻟﺘﻲ ﺗﻐﺎدر ﻣﻨﺒﻊ ﺿﻮﺋﻲ آﺎﻟﻨﺠﻢ ﺗﻨﺘﺸﺮ ﺑﺎﻟﺘﺴﺎوي ﻓﻲ ﺟﻤﻴﻊ اﻻﺗﺠﺎهﺎت . ﻓﺈن ﻋﺪد اﻟﻔﻮﺗﻮﻧﺎت اﻟﺘﻲ ﺗﻤﺮ ﻋﺒﺮ ﻣﻨﻄﻘﺔ ﻣﻌﻴﻨﺔ ﻳﺼﺒﺢ أﺻﻐﺮ آﻠﻤﺎ ازداد ﺑﻌﺪ اﻟﻤﻨﺒﻊ ﺿﻮﺋﻲ، ﺗﻨﺘﺸﺮ اﻟﻔﻮﺗﻮﻧﺎت اﻟﺘﻲ ﺗﺼﻞ إﻟﻴﻪ ﺑﺼﻮرة أوﺳﻊ وﺑﺬﻟﻚ ﻳﺼﻞ اﻟﻘﻠﻴﻞ ﻣﻨﻬﺎ إﻟﻰ ﻋﻴﻨﻪ، ﻓﻴﺒﺪوا اﻟﻤﻨﺒﻊ أﻗﻞ ﺳ ﻄ ﻮ ﻋ ﺎً . إ ذ اً ﻳﺒﺪو اﻟﻤﻨﺒﻊ ﺧ ﺎ ﻓ ﺘ ﺎً ﻷن ﻣﻌﻈﻢ ﺿﻮﺋﻪ ﻗﺪ اﻧﺘﺸﺮ ﻓﻲ اﺗﺠﺎهﺎت ﻻ ﺗﺼﻞ أ ﺑ ﺪ اً إﻟﻰ ﻋﻴﻦ اﻟﺮاﺻﺪ . ﻋﻨﺪ ﻣﺴﺎﻓﺔ d ﻣﻦ ﻣﻨﺒﻊ ﺿﻮﺋﻲ ﻳﻨﺘﺸﺮ ﺳﻄﻮع اﻟﻤﻨﺒﻊ L ﻋﻠﻰ آﺮة L ﻧﺼﻒ ﻗﻄﺮهﺎ d. ﻳﻌﻄﻰ ﺳﻄﺢ اﻟﻜﺮة ﺑﺎﻟﻌﻼﻗﺔ (4πd2)، وﺑﺬﻟﻚ ﻳﻜﻮن اﻟﻠﻤﻌﺎن اﻟﻈﺎهﺮي B اﻟﺬي ﻧﺮﺻﺪﻩ B = L/ 4πd2 2- Optical map of spiral structure The optical maps of spiral structure must be viewed with caution because interstellar dust restricts where and how far we can look from the sun and how well we can estimate distances to optical tracers. Dust decreases the measured flux, and we would estimate a larger distance than actual distance. It’s futile to draw a master map from optical data alone, which works out to distances of only some 15000 ly. Despite discord about details, most optical astronomers concur on at least three major arm segments spaced about 7000ly apart. The Galaxy appears to have a spiral structure with much irregularity in the general pattern, which may consist of two or four spiral arms – but we cannot tell for certain yet which is correct. 3-Spiral structure from radio observations • The prime drawback to optical mapping of the Galaxy is obscuration from interstellar dust. Radio observations do not have this handicap because dust does not stop radio waves. • The 21-cm line from hydrogen, which comes from the concentrations of neutral hydrogen (H I) clouds in the spiral arms, is useful as a spiral tracer. • The best radio tracer seems to be the carbon monoxide millimeter-line emission from giant molecular clouds (GMCs). • Radio observations of CO in dense molecular clouds provide an excellent tracer of the arms, and show an extension of the Sagaittarius arm at about l = 300º. Radio observations can also be made of HII clouds, and this enables their loca- tions to be mapped well beyond the limit of optical visibility Densities of HI and CO gas as function of distance from galactic centre. Note that HI extends out to ~16 kpc, but CO only to about 9 kpc, the distance of the Sun from centre. Radio observations help map the galactic disk • Looking for 21-cm wavelengths of light … • emitted by interstellar hydrogen • as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts • this allows the interstellar hydrogen to be mapped Measuring Radial Velocity • We can measure the Doppler shift of emission or absorption lines in the spectrum of an astronomical object. • We can then calculate the velocity of the object in the direction either towards or away from Earth. (radial velocity) Measuring Rotational Velocity 4- 21-cm Data and the spiral structure • The transition from neutral hydrogen at 21 cm, combined with radial velocity variations from differential galactic rotation, allows us to deduce the spiral-arm-structure in the galactic plane. • From the galactic rotation curve, the distance to concentrations of neutral hydrogen may be found from the observed Doppler-shifted 21 cm line profiles. • The distance estimation by this method is based on two assumptions: (1) differential galactic rotation and (2) circular galactic orbits for the gas near the galactic plane. This is one 21cm profile corresponds to the line of sight at l=48o, and it consists of three peaks at radial velocities of 55, 15 and -55 km/s. Clouds A, B, and C are moving away from the “recession” (R < R-sun), while cloud D is moving to the sun “approaching” (R > R-sun). Cloud A has the greatest angular speed and is moving fastest away from the Sun. A has higher density of hydrogen, so appears with the highest intensity. Clouds B and C are moving at about the same angular speed, greater than the Sun's angular speed. Cloud D is outside the solar distance, so has slower angular speed, and also has the lowest hydrogen density. HI spiral arms in the outer Galaxy (and elsewhere) A galactic plane 21-cm HI map is based on the Doppler shift of the HI clouds and the intensity of the emission from the clouds to locate the HI in the Galaxy A Map of the Milky Way Based on 21-cm wavelength light • By combining 21cm data from both the northern and southern mapping hemispheres, we can construct a schematic picture of the neutral hydrogen distribution in the spiral arms of our galaxy • The spiral structure is poorly determined near l = 0o and l = 180o, toward the galactic center and anti- center respectively. • Circularly orbiting hydrogen clouds in those directions should exhibit no radial velocity; hence, we cannot determine the distances to such clouds 5- H II regions, OB associations and GMCs tracers • The spiral arms include in addition to neutral hydrogen (H I), ionized hydrogen (H II).
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